For safety purposes, an increasing number of motor vehicles possess detection systems including electronic boxes mounted on each of the wheels of the vehicle, enclosing sensors dedicated to measuring parameters such as the radial acceleration of the wheel, and the pressure and the temperature of the tire fitted to this wheel.
These monitoring systems are conventionally equipped with electronic boxes (also called ‘wheel units’) mounted on each of the wheels of the vehicle and incorporating, in addition to the aforementioned sensors, a microprocessor, a memory and a radiofrequency transmitter, on the one hand, and with a central unit (mounted on the vehicle) for receiving the signals transmitted by the radiofrequency transmitters of each wheel, including an electronic control unit (or ECU) incorporating a radiofrequency receiver connected to an antenna, on the other hand.
Besides measuring the usual parameters that are intended to supply the driver with direct information regarding operating parameters of the wheels, it has also proved of interest to supply additional information, including characteristic data of the footprint of the tires, which enable the central unit in particular to estimate the load applied to each of the wheels of the vehicle or else to determine the location of the wheel units on the wheels of the vehicle. All of these functions require knowledge of the value of the radial acceleration of the wheels of a vehicle.
To measure radial acceleration, it is known to use radial acceleration sensors of microelectromechanical system type (also called ‘MEMS’), in particular piezoelectric accelerometers that are well known per se.
However, beyond a certain speed of the vehicle, such radial acceleration sensors are subject to saturation, making them impossible to use. Generally, such saturation sets in at a maximum radial acceleration of 350 G, corresponding to a varying speed of the vehicle, depending on the arrangement of the acceleration sensor in the wheel and also depending on the diameter of the latter. Thus, such saturation of the acceleration sensor may set in at speeds varying between 110 km/h and 200 km/h. In the present disclosure, the saturation speed under consideration of 130 km/h is therefore purely illustrative and in no way limiting. Thus, in the proximity of or beyond this threshold, radial acceleration measurements are unavailable, making the abovementioned monitoring, location or load-monitoring functions impossible, or even making it impossible to monitor the wear of the tires. Specifically, at high speeds, with the accelerometer being saturated, the tire footprint detection signal, which is essential for implementing these functions, is disabled.
This problem of saturation of the radial acceleration sensors is known and, to rectify it, it has been proposed to replace the radial acceleration sensor with a tangential acceleration sensor (see for example document US 2014/0195107 which is incorporated by reference), but this tangential acceleration sensor does not allow the system for monitoring the pressure of the tires to perform the abovementioned functions, these requiring radial acceleration measurements.